Conventionally, TN (Twisted Nematic)-type and STN (Super-Twisted Nematic)-type liquid crystal display elements which use Nematic liquid crystal are known. However, in these liquid crystal display elements, since a responding speed of an electro-optical effect is slow, namely, ms order, when high-speed driving is tried to be executed, disorder of an image occurs, and contrast is lowered. For this reason, these conventional liquid crystal display elements have a limit in a capacity of display, so there arises a problem that they are not suitable for displaying a dynamic image. Therefore, in recent years, as a next-generation liquid crystal display -element, a practical application of a liquid crystal display element using ferroelectric or antiferroelectric liquid crystal is being examined.
In 1975, R. B. Meyer and the others have predicted that if optically active molecules have a dipole moment in a direction which is perpendicular to a molecular major axis, liquid crystal shows ferroelectricity in a chiral smectic C phase (SmC* phase), and have synthesized DOBAMBC (2-methylbutyl p- p-(decyloxybenzylidene) -amino!-chinnamate), and first succeeded in confirming ferroelectricity in liquid crystal (see R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller: J. Phys. (Paris) 36 (1975) L69).
The following describes a structure of the SmC* phase which shows ferroelectricity as one example of the smectic liquid crystal phase. In the SmC* phase, a position of the center of gravity of the liquid crystal molecules in a layer is disorder, but as shown by a cone 101 in FIG. 12(a), a major axis of the liquid crystal molecules (director 102) is tilted only by a constant angle .theta. with respect to a layer normal line z which is a normal line of a layer surface 103 dividing a smectic layer. The tilting direction of the director 102 slightly shifts from a layer to a layer, and as a result, alignment of the liquid crystal molecules has a helical structure. A helical pitch is about 1 .mu.m, so it is much larger than a layer intervals of about 1 nm.
A phase having such a molecular arrangement is confirmed not only in ferroelectric liquid crystal but also in antiferroelectric liquid crystal (see A. D. L. Chandani, T. Hagiwara, Y. Suzuki, Y. Ouchi, H. Takezoe and A. Fukuda: Jpn. J. Appl. Phys. 27(1988) L729.). In some antiferroelectric liquid crystal, when optical purity is changed, SmC* phase appears, and in some R-configuration or S-configuration whose optical purity is 100% such as MHPOBC (4-(1-methylheptyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate), the SmC* phase appears.
Clark and Lagerwall discovered that when a cell thickness became not more than about 1 .mu.m (equivalent to the helical pitch), the helical structure disappeared, and as shown in FIG. 12(b), molecules 104 on each layer were in either of bi-stable states according to an electric field to be applied, and suggested a surface stabilized ferroelectric liquid crystal display element (SSFLC). The SSFLC is disclosed in Japanese Unexamined Patent Publication No. 56-107216/1981 (Tokukaisho 56-107216), U.S. Pat. No. 4367924, etc. In FIG. 12(b), a direction of an electric field applied to the molecules 104 is a direction which is perpendicular to a paper surface and is directed from the back side of paper to the front side. All the electric dipole moments of the molecules 104 are arranged in the direction of the electric field as shown in each molecule in FIG. 12(b).
The following describes an operating principle on reference to FIG. 13. As mentioned above, the molecules 104 of SSFLC which are formed as thin cells, as shown in FIG. 13, are brought into one of two stabilized state, i.e. states A and B according to the direction of an electric field to be applied. In the state A shown in the drawing, the direction of the electric field applied to the molecules 104 is perpendicular to the paper surface in the drawing and is directed from the front side of the paper to the back side. In the state B, the direction of the electric field is perpendicular to the paper surface and is directed from the back side of the paper to the front side.
For this reason, by positioning a SSFLC cell between two polarizers which perpendicularly cross each other so that a molecular major axis in the state B is parallel with a direction of one of the polarizers (direction 111 shown by an arrow in the drawing), in the state A, a light is transmitted so that a bright state is obtained, and in the state B, a light is blocked so that a deep state is obtained. Namely, a black-and-white display can be executed by switching the direction of the applied electric field.
In SSFLC, since the spontaneous polarization and the electric field interact with each other, and thus driving torque occurs, the high speed response of .mu.s order to the electric field is possible unlike switching by means of dielectric anisotropy in the normal Nematic liquid crystal. Moreover, when the SSFLC is once switched to one of the bi-stable states, even if the electric field disappears, the SSFLC has a so-called memory characteristic that keeps the state. Therefore, it is not always necessary that a voltage is applied thereto.
As mentioned above, in the SSFLC-type liquid crystal display element, display contents can be written at a high speed per one scanning line by utilizing the high-speed response and the memory characteristic. As a result, a simple-matrix driven display having a large capacity can be realized, and the application of this display to a wall-hanging-type TV is expected.
The liquid crystal element using ferroelectric liquid crystal can strictly realize only two-tone display i.e. bright and deep display due to bi-stability of liquid crystal molecules in the SmC* phase, but an arrangement that makes the tone display possible in a certain degree by utilizing high-speed modulation of electric field to be applied and dot matrix method is suggested. However, such conventional arrangements require a complicated arrangement of a driving system and a complicated panel manufacturing process, thereby increasing the manufacturing cost, etc.
In addition, for example, Japanese Unexamined Patent Publication No. 6-194635/1994 (Tokukaisho 6-194635) discloses a technique for forming a structure obtained by adding non-reactant chiral liquid crystal molecules into a three-dimensional anisotropic network structure composed of a polymer. This makes it possible to stabilize minute adjacent domains having opposite polarization directions to each other by means of the network structure and maintain a half tone even when an electric field does not exist.
However, the technique disclosed in Japanese Unexamined Patent Publication No. 6-194635/1994 (Tokukaisho 6-194635) has a disadvantage that a domain size is not uniform and thus areas of the domains cannot be kept constant at the time of applying a voltage. Since the domain size is considerably larger than an actual pixel size (about 0.3 by 0.3 mm square), it is practically difficult to execute tone display using this technique.
In addition, when a quantity of chiral material to be added to a liquid crystal material as a host is increased, alignment of liquid crystal becomes worse, and the display quality is deteriorated.
In addition, it is a serious problem which should be solved to the ferroelectric liquid crystal display element to improve shock resistance. Namely, the SSFLC-type liquid crystal display element is easily affected by a pressure from outside and an electrical shock, and thus the alignment is easily disordered. On the contrary, a method of forming a spacer wall to a substrate has been suggested, but this method causes various problems in the panel manufacturing process, so this method has not been put into practical use.
Furthermore, the ferroelectric liquid crystal element is capable of high-speed driving more easily than TN-type liquid crystal element, etc. However, in order to make it possible to apply the ferroelectric liquid crystal element to a flat panel display with a wide screen such as a wall-hanging-type TV, higher responding speed is desired. Moreover, it is preferable to suppress a driving voltage lower in order to decrease power consumption and suppress a calorific value.